The present invention relates to a color cathode-ray tube and, more particularly, to a color cathode-ray tube including an electron gun which, in operation, emits three electron beams in line in a horizontal direction toward a fluorescent screen.
Owing to their fine picture-reproducing property, color cathode-ray tubes, such as color picture tubes or display tubes, have been extensively used for receiving TV broadcast programs and as monitors for data processing equipment.
The color cathode-ray tube of this type includes a panel portion having a face plate forming a fluorescent screen on the inner surface thereof, a neck portion containing an electron gun structure for emitting electron beams onto the fluorescent screen, and an evacuated envelope having at least a funnel portion for connecting said panel portion to said neck portion.
FIG. 39 is a diagram schematically illustrating, in cross section, the constitution of a shadow mask-type color cathode-ray tube to which the present invention is adapted, and wherein reference numeral 20 denotes a face plate portion, 21 denotes a neck portion, 22 denotes a funnel portion for connecting the panel portion to the neck portion, 23 denotes a fluorescent screen which constitutes an image display screen on the inner surface of the face plate, 24 denotes a shadow mask which operates as a color-selection electrode, 25 denotes a mask frame which constitutes a shadow mask structure to hold the shadow mask, 26 denotes an inner shield for shielding external magnetism, 27 denotes a suspension spring mechanism by which the shadow mask structure is suspended by studs that are located on the inner wall of the face plate, 28 denotes an electron gun accommodated in the neck portion for emitting three electron beams Bs (x2) and Bc in line, 29 denotes a deflection device for deflecting the electron beams in the horizontal and vertical directions, and reference numeral 30 denotes a magnetic device for correcting color purity or centering.
In the diagramed constitution, an evacuated envelope is constituted by the face plate 20, neck portion 21 and funnel portion 22. Three electron beams Bc and Bs (x2) emitted in line from the electron gun 28 are deflected in the horizontal and vertical directions by a deflecting magnetic field formed by the deflecting device 29 so as to two-dimensionally scan the fluorescent screen 23. Here, symbol Bc denotes a center beam, and Bs denotes a side beam.
The three electron beams Bc and Bs (x2) are each modulated by color signals of red (side beam Bs), green (center beam Bc) and blue (side beam Bs), subjected to the color selection through beam passage holes of the shadow mask 24 arranged in front of the fluorescent screen 23, and impinge upon fluorescent mosaics of red, green and blue colors that constitute the fluorescent screen 23 to reproduce a desired color image.
FIG. 40 is a horizontal sectional view of an in-line electron gun mounted in a conventional color cathode-ray tube, wherein reference numeral 1 denotes cathodes, 2 denotes a control electrode, 3 denotes an accelerating electrode, 4 denotes a focus electrode assembly, 5 denotes an anode, and reference numeral 6 denotes a shield cup. Reference numeral 41 denotes a first focus electrode, 42 denotes a second focus electrode, and the focus electrode assembly 4 is constituted by these focus electrodes. Reference numerals 411 and 421 denote plate electrodes constituting an electrostatic quadrupole lens.
Thermoelectrons emitted from the heated cathodes 1 are accelerated toward the control electrode 2 due to a potential applied to the accelerating electrode 3, whereby three electron beams are formed. The three electron beams pass through apertures in the control electrode 2 and pass through apertures in the accelerating electrode 3. Then, the three electron beams are focused to some extent by a prefocus lens formed between the accelerating electrode 3 and the first focus electrode 41 prior to entering into the main lens formed between the second focus electrode 42 and the anode 5, the beams being fed to the main lens while being accelerated by the potential of the focus electrode 4. The three electron beams are focused by the main lens formed between the second focus electrode 42 and the anode 5 on the fluorescent screen to form a projection spot.
The first focus electrode 41 is supplied with a predetermined voltage (Vf1) 7, and the second focus electrode 42 is supplied with a dynamic voltage (Vf2+dVf) 8 that changes in synchronism with a change in the deflection angle for scanning the electron beams on the screen. Symbol Eb denotes an anode voltage.
The intensity of the main lens is changed depending upon the deflection angle of the electron beam, thereby to correct the curvature of the image field. Any astigmatism is corrected by the electrostatic quadrupole lens constituted by the vertical plate electrode 411 and the horizontal plate electrode 421 mounted on the first focus electrode 41 and on the second focus electrode 42, in order to control the focusing distance of the electron beam and the shape of the beam spot, thereby obtaining a good focus on the screen at all times.
In the practical cathode-ray tube, however, a desired voltage is not obtained at the periphery of the screen due to a limitation on the drive circuit of the dynamic voltage 8, and a favorable beam spot is not obtained.
Japanese Patent Laid-Open No. 43532/1992 (U.S. Pat. No. 5,212,423) discloses a method which suppresses the amount of change in the dynamic voltage that varies in synchronism with the deflection angle, in order not to increase the diameter of the electron beams.
FIG. 41 is a horizontal sectional view illustrating the constitution of a conventional in-line type electron gun disclosed in the above-mentioned publication, wherein a focus electrode assembly 4 is constituted by a first focus electrode 43, a second focus electrode 44, a third focus electrode 45 and a fourth focus electrode 46. Reference numeral 442 denotes horizontal correction plate electrodes constituting the electrostatic quadrupole lens, and 454 denotes vertical correction plate electrodes for constituting an electrostatic quadrupole electrode. The same reference numerals as those of FIG. 40 denote portions having the same functions.
As shown, the focus electrode assembly 4 is divided into a plurality of electrode groups 43, 44, 45 and 46, and the electrostatic quadrupole lens is constituted by the horizontal plate electrodes 442 and vertical plate electrodes 454 among these focus electrode groups. Among these focus electrode groups there is further formed at least an electron lens which exhibits a strong focusing force in both the horizontal direction and the vertical direction. This electron lens (hereinafter referred to as a lens for correcting the field curvature) has a function for correcting the curvature of the image field, which corresponds to the inner surface of the panel.
Furthermore, a main lens formed between the opposing surfaces of the fourth focus electrode 46 and the anode 5 produces a strong astigmatism for vertically deforming the sectional shape of the electron beams. Here, in the conventional electron gun described above, a method of applying DC components (Vf1, Vf2) of two focus voltages must be changed in order to impart the action of the lens for correcting the field curvature to the electron lens that exhibits a strong focusing force in both the horizontal direction and the vertical direction. However, the method of applying a dynamic voltage is the same.
That is, so far, the two DC focus voltages have nearly equal values, and the dynamic voltage increases accompanying an increase in the amount of deflection of the electron beams. In the electron gun shown in FIG. 41, on the other hand, the one DC focus voltage (Vf1) is considerably greater than the other DC focus voltage (Vf2), and a difference in the voltage is set to be at least greater than a maximum value of the dynamic voltage (dV1).
Therefore, when the dynamic voltage increases, i.e., when the amount of deflection of the electron beam increases, the potential difference decreases in the lens in which the focusing force increases in both the horizontal direction and the vertical direction, and the intensity of the lens decreases. Therefore, the force for focusing the electron beam decreases when the electron beam is deflected, and the field curvature is corrected.
In FIG. 42, symbol 1H denotes a horizontal period and 1V denotes a vertical period.
Since the action for correcting the field curvature which is, so far, based on the main lens only is reinforced by at least a lens for correcting the field curvature, it is possible to lower the dynamic voltage for correction.
Furthermore, the main lens through which the electron beams of the outer sides pass, is axially asymmetric and works to deflect the electron beams of the outer sides toward the side of the central electron beam so that they are in agreement with the center beam on the fluorescent screen (so-called STC: static convergence). With the three electron beams in agreement on the fluorescent screens, the pictures of R, G and B colors due to the electron beams are properly superposed to display a color picture.
Due to the magnetic field generated by the deflection yoke, furthermore, the three electron beams are scanned on the fluorescent screen to display the picture. A self-convergence deflection yoke is used as the deflection yoke.
The shape of the inner surface of the panel (shape of the image field) is not spherical with respect to the center of deflection. When the magnetic field of the deflection yoke is symmetrical, therefore, the three electron beams that are coincident on the center of the fluorescent screen become out of coincidence when they are deflected. Therefore, the self-convergence deflection yoke so deflects the magnetic field that the magnetic field in the horizontal direction is distributed in a pin-cushion manner and the magnetic field in the vertical direction is distributed in an asymmetrical barrel shape, so that the three electron beams are coincident on the whole fluorescent screen.
Japanese Patent Laid-Open No. 72546/1990 (U.S. Pat. No. 4,851,741) discloses means for improving the convergence of the three electron beams on the fluorescent screen.
This is the technology for improving the phenomenon in which the focus voltage applied to the focus electrode (fourth focus electrode 46 in FIG. 41) opposed to the anode 5 to form a main lens, varies in synchronism with a change in the deflection angle for scanning the electron beams on the screen, causing the intensity of the main lens formed by the anode 5 and the fourth focus electrode 46 to be changed and, hence, causing the STC action of the main lens to be varied.
In other words, the STC is varied by the electrostatic quadrupole lens in a direction opposite to the change in the STC caused by the main lens, so that variations in the STC caused by the lenses are canceled by each other.
According to this method, the STC is changed by the electrostatic quadrupole lens, and the astigmatism is corrected simultaneously with the STC. Therefore, a high degree of technology is required for optimizing the structure for simultaneously satisfying the STC and the correction of astigmatism. Besides, when the sizes of the electrodes constituting the electrostatic quadrupole lens undergo a change, both the STC and the correction of astigmatism are changed, and the screen resolution is deteriorated. Therefore, strict accuracy is required for the electrodes constituting the electrostatic quadrupole lens.
According to Japanese Patent Laid-Open No. 31332/1996, the STC is varied by the electrostatic quadrupole lens and, at the same time, the astigmatism is corrected. Besides, the intensity of the lens is varied by the electrostatic quadrupole lens for the center electron beam and by the electrostatic quadrupole lens for the side electron beams.
This method, too, requires a high degree of technology for optimizing the electrode structure for satisfying both the STC and the correction of astigmatism. Besides, when the sizes of the electrodes constituting the electrostatic quadrupole lens undergo a change, both the STC and the correction of astigmatism are changed, and the screen resolution is deteriorated. Therefore, strict accuracy is required for the electrodes constituting the electrostatic quadrupole lens.
According to Japanese Patent Laid-Open No. 31333/1996 (U.S. Pat. No. 5,608,284) filed by the present applicant, the action for canceling a change in the STC caused by the main lens (action for correcting a change in the STC) is exhibited by a lens for correcting the field curvature, but not by the electrostatic quadrupole lens.
The electron gun disclosed in the above-mentioned Japanese Patent Laid-Open No. 31333/1996 (U.S. Pat. No. 5,608,284) can be easily produced since the lens for correcting the field curvature exhibits an action for canceling a change in the STC caused by the main lens.
According to the electron gun disclosed in Japanese Patent Laid-Open No. 31333/1996 (U.S. Pat. No. 5,608,284), however, no attention has been given in regard to maintaining balance between the intensity of the lens for correcting the field curvature for the center electron beam and the intensity of the lens for correcting the field curvature for the outer electron beams. The present inventors have discovered the fact that the center electron beam and the outer electron beams lose balance so as to form spots.
In order to cancel a change in the STC caused by the main lens, the electrodes belonging to the first type of focus electrode group forming the lens for correcting the field curvature and the electrodes belonging to the second type of focus electrode group, have three electron beam passage holes that are arranged in the horizontal direction. Here, the substantial centers of the outer electron beam passage holes in an electrode belonging to the second type of focus electrode group are deviated with respect to the substantial centers of the outer electron beam passage holes in the electrode belonging to the first type of focus electrode group, in order to deflect the outer electron beams toward the center electron beam with an increase in the deflecting amount of the electron beams.
Since the substantial centers of the outer electron beam passage holes in the electrode belonging to the second type of focus electrode group are deviated relative to the substantial centers of the outer electron beam passage holes in the electrode belonging to the first type of focus electrode group, the lens for correcting the field curvature exhibits different intensities for the outer electron beams passing through the holes and for the center electron beam passing through the hole.
Therefore, a difference develops in a ratio of the diameter of the outer electron beams incident on the main lens in the horizontal direction to the diameter thereof in the vertical direction, and in a ratio of the diameter of the center electron beam incident on the main lens in the horizontal direction to the diameter thereof in the vertical direction, with an increase in the difference between a first focus voltage applied to the first type of focus electrode group and a second focus voltage applied to the second type of focus electrode group.
At the center of the screen where the difference becomes a maximum between the first focus voltage applied to the first type of focus electrode group and the second focus voltage applied to the second type of focus electrode group, the difference becomes a maximum between the ratio of the diameter of the outer electron beam incident on the main lens in the horizontal direction to the diameter thereof in the vertical direction and the ratio of the diameter of the center electron beam incident on the main lens in the horizontal direction to the diameter thereof in the vertical direction.
The difference in the ratios of the diameter in the horizontal direction to the diameter in the vertical direction between the center electron beam and the outer electron beams incident on the main lens causes the center electron beam and the outer electron beams to lose balance, resulting in the formation of spots.
The object of the present invention is to provide a color cathode-ray tube, which avoids the effects of the above-mentioned prior art and which is equipped with an electron gun which makes it possible to obtain an excellent resolution over the whole screen.
The present invention relates to a color cathode-ray tube comprising at least an electron beam-generating portion for generating three electron beams which are arranged in a horizontal direction and are controlled, an electron gun having a main lens for focusing the three electron beams generated by said electron beam-generating portion on a fluorescent screen, and a deflection yoke for scanning said three electron beams on the fluorescent screen in both the horizontal direction and the vertical direction.
A main lens in the electron gun comprises an anode to which an anode voltage is applied, a first type of focus electrode group to which is applied a first focus voltage which is a predetermined voltage, and a second type of focus electrode group to which a second focus voltage is applied, the electrodes belonging to the second type focus electrode group neighboring the anode. The second focus voltage is obtained by superposing a dynamic voltage that changes depending upon the amount of deflection of the electron beams on a predetermined voltage, which is lower than the first focus voltage.
Between the first type of focus electrode group and the second type of focus electrode group, there are formed at least two electron lenses, i.e., a lens for correcting the field curvature which exhibits an increasing force for focusing the three electron beams in both the horizontal direction and the vertical direction with an increase in the potential difference between the first focus voltage and the second focus voltage, and an electrostatic quadrupole lens which exhibits a large force for focusing said three electron beams in either the horizontal direction or the vertical direction and a large force for diverging the three electron beams in the other of these directions.
Three electron beam passage holes arranged in a horizontal direction are formed in the electrode belonging to the first type of focus electrode group and in the electrode belonging to the second type of focus electrode group. The three electron beam passage holes are so arranged that the centers of the outer electron beam passage holes of the electrode belonging to the second type of focus electrode group are deviated on a horizontal plane relative to the centers of the outer electron beam passage holes of the electrode belonging to the first type of focus electrode group. Furthermore, the electrostatic quadrupole lens exhibits dissimilar intensities for the outer electron beams and for the center electron beam.
In order to accomplish the above-mentioned objects, the features of the present invention resides in the below-mentioned constitutions (1) to (17).
(1) A color cathode-ray tube comprising at least an electron beam-generating portion for generating three electron beams that are arranged in a horizontal direction and are controlled, an electron gun having a main lens portion for focusing the three electron beams generated by said electron beam-generating portion on a fluorescent screen, and a deflection yoke for scanning said three electron beams on the fluorescent screen in both the horizontal direction and the vertical direction; wherein
an electrode group for forming said electron beam-generating portion and an electrode group for forming said main lens have center beam passage holes of which the center axes are in agreement with each other;
the main lens of said electron gun includes an anode to which an anode voltage is applied, a first type of focus electrode group to which a first focus voltage is applied, and a second type of focus electrode group to which a second focus voltage is applied;
an electrode belonging to the second type of focus electrode group neighbors said anode, and said second focus voltage is obtained by superposing, on a predetermined voltage, a dynamic voltage that changes depending upon the amount of deflection of the electron beams;
between the first type of focus electrode group and the second type of focus electrode group are formed at least two electron lenses, i.e., a lens for correcting the field curvature which exhibits an increasing force for focusing the three electron beams in both the horizontal direction and the vertical direction with an increase in the potential difference between the first focus voltage applied to said first type of focus electrode group and the second focus voltage applied to said second type of focus electrode group, and an electrostatic quadrupole lens which exhibits a large force for focusing said three electron beams in either the horizontal direction or the vertical direction and a large force for diverging the three electron beams in the other of these directions;
among said three electron beam passage holes arranged in a horizontal direction in the electrode belonging to said first type of focus electrode group and in the electrode belonging to said second type of focus electrode group forming said lens for correcting the field curvature, the outer electron beam passage holes are so arranged that the substantial centers of the outer electron beam passage holes in the electrode belonging to said second type of focus electrode group are deviated on a horizontal plane relative to the substantial centers of the outer electron beam passage holes in the electrode belonging to the first type of focus electrode group, so that the outer electron beams are deflected toward the center electron beam with an increase in the deflecting amount of the electron beams; and
said electrostatic quadrupole lens has an electrode constitution which exhibits different intensities for the outer electron beams and for the center electron beam.
(2) A color cathode-ray tube of constitution (1), wherein the substantial centers of said outer electron beam passage holes in the electrode belonging to said first type of focus electrode group are deviated toward the center electron beam relative to the substantial centers of the outer electron beam passage holes in the electrode belonging to said second type of focus electrode group, said first type of focus electrode group forming a lens for correcting the field curvature that works to deflect the outer electron beams toward the center electron beam with an increase in the deflecting amount of the electron beams, and said electrostatic quadrupole lens has an electrode constitution which exhibits a stronger intensity for the outer electron beams than for the center electrode beam.
(3) A color cathode-ray tube of constitution (2), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electrode beam and the electrode beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electrode beams on both sides from the upper and lower sides in the vertical direction, and among said vertical plate electrodes, the vertical plate electrodes sandwiching the electron beams on both sides have a gap in the horizontal direction which is smaller than a gap between the vertical plate electrodes sandwiching the center electron beam in the horizontal direction.
(4) A color cathode-ray tube according to constitution (2), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electrode beam and the electrode beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electrode beams on both sides from the upper and lower sides in the vertical direction, and among said horizontal plate electrodes, the horizontal plate electrodes sandwiching the electron beams on both sides have a gap in the vertical direction which is smaller than a gap between the horizontal plate electrodes sandwiching the center electron beam in the vertical direction.
(5) A color cathode-ray tube of constitution (2), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electrode beam and the electrode beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electrode beams on both sides from the upper and lower sides in the vertical direction, and wherein at least either said vertical plate electrodes or said horizontal plate electrodes are so constituted that the length of the plate electrodes sandwiching the electron beams of both sides in the axial direction of the tube is greater than the length of the plate electrodes sandwiching the center electron beam in the axial direction of the tube.
(6) A color cathode-ray tube of constitution (2), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electrode beam and the electrode beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the upper and lower sides in the vertical direction, and wherein at least either said vertical plate electrodes or said horizontal plate electrodes sandwiching the electron beams of both sides in the vertical direction or in the horizontal direction, have a width which is larger than the width of the plate electrodes sandwiching the center electron beam.
(7) A color cathode-ray tube of constitution (2), wherein said electrostatic quadrupole lens is constituted by electron beam passage holes in the electrode belonging to said first type of focus electrode group, and at least a pair of horizontal plate electrodes formed on the electrode belonging to said second type of focus electrode group at end surfaces opposed to the electrode that belongs to said first type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the upper side and the lower side in the vertical direction, and, among the electron beam passage holes in the electrode belonging to said first type of focus electrode group, the ratio of the diameter of the outer electron beam passage holes in the vertical direction to the diameter thereof in the horizontal direction is greater than the ratio of the diameter of the center electron beam passage hole in the vertical direction to the diameter thereof in the horizontal direction.
(8) A color cathode-ray tube of constitution (2), wherein the ratio (diameter in the vertical direction/diameter in the vertical direction) of the diameter in the vertical direction of the outer electron beam passage holes among the electron beam passage holes in the electrode belonging to said first type of focus electrode group forming the electrostatic quadrupole lens to the diameter thereof in the horizontal direction is greater than the ratio of the diameter of the center electron beam passage hole in the horizontal direction to the diameter thereof in the vertical direction.
(9) A color cathode-ray tube of constitution (2), wherein the ratio of the diameter in the vertical direction of the outer electron beam passage holes among the electron beam passage holes in the electrode belonging to said second type of focus electrode group forming the electrostatic quadrupole lens to the diameter thereof in the horizontal direction is smaller than the ratio of the diameter of the center electron beam passage hole in the vertical direction to the diameter thereof in the horizontal direction.
(10) A color cathode-ray tube of constitution (1), wherein the substantial centers of said outer electron beam passage holes in the electrode belonging to said second type of focus electrode group are deviated toward a direction opposite to the center electron beam relative to the substantial centers of the outer electron beam passage holes in the electrode belonging to said first type of focus electrode group, said second type of focus electrode group forming a lens for correcting the field curvature that works to deflect the outer electron beams toward the center electron beam with an increase in the amount of deflection of the electron beams, and said electrostatic quadrupole lens has an electrode constitution which exhibits a weaker intensity for the outer electron beams than for the center electrode beam.
(11) A color cathode-ray tube of constitution (10), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the upper and lower sides in the vertical direction, and among said vertical plate electrodes, the vertical plate electrodes sandwiching the electron beams on both sides have a gap in the horizontal direction which is larger than a gap between the vertical plate electrodes sandwiching the center electron beam in the horizontal direction.
(12) A color cathode-ray tube according to constitution (10), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electrode beams on both sides from the upper and lower sides in the vertical direction, and among said horizontal plate electrodes, the horizontal plate electrodes sandwiching the electron beams on both sides having a gap in the vertical direction which is larger than a gap between the horizontal plate electrodes sandwiching the center electron beam in the vertical direction.
(13) A color cathode-ray tube of constitution (10), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electrode beams on both sides from the upper and lower sides in the vertical direction, and wherein at least either said vertical plate electrodes or said horizontal plate electrodes are so constituted that the length of the plate electrodes sandwiching the electron beams of both sides in the axial direction of the tube is smaller than the length of the plate electrodes sandwiching the center electron beam in the axial direction of the tube.
(14) A color cathode-ray tube of constitution (10), wherein said electrostatic quadrupole lens is constituted by vertical plate electrodes formed on the electrode belonging to said first type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the two sides in the horizontal direction, and at least a pair of horizontal plate electrodes formed on the electrode belonging to the second type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the upper and lower sides in the vertical direction, and wherein at least either said vertical plate electrodes or said horizontal plate electrodes sandwiching the electron beams of both sides in the vertical direction or in the horizontal direction, have a width which is smaller than the width of the plate electrodes sandwiching the center electron beam.
(15) A color cathode-ray tube of constitution (10), wherein said electrostatic quadrupole lens is constituted by electron beam passage holes in the electrode belonging to said first type of focus electrode group, and at least a pair of horizontal plate electrodes formed on the electrode belonging to said second type of focus electrode group at end surfaces opposed to the electrode that belongs to said first type of focus electrode group to sandwich the center electron beam and the electron beams on both sides from the upper side and the lower side in the vertical direction, and, among the electron beam passage holes in the electrode belonging to said first type of focus electrode group, the ratio of the diameter of the outer electron beam passage holes in the vertical direction to the diameter thereof in the horizontal direction is smaller than the ratio of the diameter of the center electron beam passage hole in the vertical direction to the diameter thereof in the horizontal direction.
(16) A color cathode-ray tube of constitution (10), wherein the ratio of the diameter in the vertical direction of the outer electron beam passage holes among the electron beam passage holes in the electrode belonging to said first type of focus electrode group forming the electrostatic quadrupole lens to the diameter thereof in the horizontal direction is smaller than the ratio of the diameter of the center electron beam passage hole in the vertical direction to the diameter thereof in the horizontal direction.
(17) A color cathode-ray tube of constitution (10), wherein the ratio of the diameter in the vertical direction of the outer electron beam passage holes among the electron beam passage holes in the electrode belonging to said second type of focus electrode group forming the electrostatic quadrupole lens to the diameter thereof in the horizontal direction is greater than the ratio of the diameter of the center electron beam passage hole in the vertical direction to the diameter thereof in the horizontal direction.
The above-mentioned constitutions make it possible to decrease the dynamic voltage, to decrease a change in the STC, to eliminate spot-like imbalance of the electron beams on the screen and, hence, to obtain a favorable resolution on the whole screen.
The effects owing to the constitutions of the present invention will become more obvious from the following description of various embodiments of the invention when considered with the accompanying drawings.